A navigation system performs travel guidance for enabling a user to easily and quickly reach the selected destination. A typical example is a vehicle navigation system where a user drives a car equipped with a navigation function. Such a navigation system detects the position of the user or user's vehicle, and reads out map data pertaining to an area at the current vehicle position from a data storage medium, for example, a CD-ROM (compact disc read-only memory), a DVD (digital versatile disc), or a hard disc. Alternatively, such map data can be provided to the user from a remote server through a communication network such as Internet. The navigation system displays a map image on a monitor screen while superimposing a mark representing the current location of the user on the map image.
When a destination is set, the navigation system starts a route guidance function for setting a guided route from the start point to the destination. To determine the guided route to the destination, the navigation system calculates and determines an optimum route to the destination based on various parameters. For example, the guided route is determined based on the shortest way to reach the destination, the route preferring freeways to surface roads, the least expensive way to the destination, the route without using toll road, or the like.
During the route guidance, the navigation system reads the nodes data from the data storage medium such as DVD and successively stores the nodes data of road segments (expressed in longitude and latitude) constituting the guided route in a map memory. In the actual traveling, the node series stored in the map memory is searched for a portion of the guided route to be displayed in a map display area of the monitor screen, and the portion of the guided route is highlighted to be clearly distinguished from other routes. When the vehicle is within a predetermined distance of an intersection it is approaching, an intersection guidance diagram (an enlarged or highlighted intersection diagram with an arrow indicating the direction in which the vehicle is to turn at the intersection) is displayed to inform a user of the desired one of roads or directions selectable at the intersection. Such route guidance by the navigation system is also given by voice instruction.
FIGS. 1A-1H show an example of overall procedure and screen display involved in the navigation system to obtain a maneuver guidance information screen. FIG. 1A shows an example of locator map screen of the navigation system when the destination is not specified. Typically, the navigation system displays a street on which the vehicle (current vehicle position VP) is running on a map image and a name of the street. Other information such as a north pointer NP, a map scale and a current time may also be illustrated on the display screen.
An example of process for specifying a destination in the navigation system through a Point of Interest (POI) search method is shown in FIGS. 1B-1F. A main menu screen such as shown in FIG. 1B displays menu items including a “Destination” menu for entering the destination. When selecting “Destination”, the navigation system displays a “Find Destination by” screen as shown in FIG. 1C for specifying an input method for selecting the destination. The “Find Destination By” screen lists various methods for selecting the destination including “Address” for specifying the city and address of the destination, “Intersection” for specifying the names of two streets which intersect with one another, and “Point of Interest (POI)” for selecting the programmed destination based on the name, category or telephone number.
When selecting, the “Point of Interest” method in FIG. 1C, the navigation system displays selection methods of point of interest (POI) either by “Place Name” or “Place Type” in FIG. 1D. The “Place Name” is to specify a name of POI, and the “Place Type” is to specify a category of POI. If the “Place Type” is selected in FIG. 1D, the navigation system lists categories of POIs as shown in FIG. 1E. The user selects a desired category of POIs from the lists.
FIG. 1F shows a screen when the user has selected a “Fast Foods” category in the example of FIG. 1E. In this example, the screen includes the name of POI type “Fast Foods” at the top and a list of names of the fast food restaurants typically sorted by distance from the current destination. The user selects a particular restaurant among the restaurant lists for route guidance. In FIG. 1G, the navigation system calculates an optimum route to the selected destination. After determining the guided route, the navigation system starts the route guidance as shown in FIG. 1H. Typically, the navigation system shows the intersection that is highlighted to show the next turn and a direction of the turn.
FIG. 2 shows an example of a display listing a selected type of POI place names, in this case, fast food restaurants. It should be noted that although the present invention is not limited to the application of finding POIs, an example of disadvantages in the conventional technology will be described for the case involved with selecting one of POIs for simplicity of explanation. In the conventional technology, such POI names are listed in the alphabetical order or in the order of distance from the user's present location. Further in the conventional technology, such POIs are evaluated one by one and sorted one by one to be listed in the specified order as noted above.
In an actual use of a navigation system, a user does not necessarily know exactly which POI such as a fast food restaurant he wants to go. The user may not have decided as to what type of food he wants to eat. In such a case, the user simply wants to go to a location where many fast food restaurants are located so that he can choose one of the fast food restaurants he wants at the location. The user may want to compare outside appearances or displays of the restaurants to see which restaurant seems to offer foods or atmosphere to his preference.
FIG. 3 shows the condition where the user may prefer to travel a longer distance to go to a location where points of interest are clustered. Reference numbers 103 indicate the locations of POIs of the selected type on the digitized image map. A reference mark 105 indicates the present location of the user. In terms of distance, the user can reach the POI at a location A which is at the shortest distance. However the user can find three POIs at a location B or six POIs at a location C although they are located farther than the location A.
The user may often prefer to go to the location B or C where two or more POIs are clustered especially when the user does not have a specific POI in mind or he wants to decide one by actually seeing it. In the conventional navigation system, the user cannot easily tell whether particular type of POIs are clustered in a particular location. Under the circumstances, there is a need in the navigation system to have a function to search POIs in terms of relative spatial density, i.e., cluster size and location.